JP7224443B2 - Clad steel plate and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a clad steel sheet and a method for manufacturing the same. In particular, it relates to a clad steel plate having a laminate of duplex stainless steel having a chemical composition containing 0.5 to 2.5% of Mo.

Duplex stainless steel contains a large amount of Cr, Mo, Ni, and N, and intermetallic compounds and nitrides are likely to precipitate. It was made from steel. For this reason, the clad steel plate with duplex stainless steel as the cladding material is made of carbon steel with a devised chemical composition so that the mechanical properties can be secured even if it is heat-treated at a high temperature of 1000 ° C or higher during manufacturing. is proposed (Patent Document 1). A technology has also been proposed for producing a duplex stainless steel clad steel sheet by controlling the hot rolling conditions to omit the heat treatment (Patent Document 2). Furthermore, a technique of reheating during hot rolling to suppress precipitation in the laminated material has also been proposed (Patent Document 3).

On the other hand, a clad steel plate of reduced alloy element type duplex stainless steel in which Ni, Mo and the like are reduced has also been proposed (Patent Document 4). Furthermore, in order to solve the problem that CaS in steel becomes the starting point of pitting corrosion and impairs the corrosion resistance of steel, a duplex stainless steel was used in which the amount of Ca and Al added was controlled to render CaS harmless against corrosion resistance. A clad steel plate has also been proposed (Patent Document 5). In alloying element-reduced duplex stainless steels, the main influencing precipitates are chromium nitrides.

JP-A-7-292445 Japanese Patent Publication No. 4-22677 Japanese Patent Publication No. 6-36993 JP 2012-180567 A JP 2018-028146 A

ISIJ Vol. 58 (2018), p1181-1183

A clad steel plate is a steel material that can obtain multiple properties by giving corrosion resistance to the stainless steel used as a cladding material and giving strength, toughness, and weldability to the base material. A clad steel plate is used in a portion where a stainless steel as a cladding material and a base material are structurally joined, and is generally used in applications where the plate thickness is large and strength and toughness are particularly required. Examples include seawater desalination equipment, tanks for transport ships, and the like. Conventionally, austenitic stainless steel has been mainly used for these applications, but recently, the number of cases where inexpensive duplex stainless steel is applied is increasing.
Therefore, there is a strong demand for a clad steel plate in which the cladding material is duplex stainless steel. When a clad steel plate is applied, the base material takes charge of the functions of strength and toughness, and the cladding material takes charge of corrosion resistance. In particular, steel members for dams and water gates in brackish water areas include sliding members such as doorstops and rails, and both slidability and corrosion resistance are sometimes required.

By the way, the inventors of the present invention have found that the corrosion resistance of a clad steel plate made of a duplex stainless steel having a chemical composition with a Mo content of 0.5 to 2.5% is a sigma (σ) phase. It was found that the precipitation of the compound mainly affected.
The sigma phase is an intermetallic compound with a high Cr content. When the sigma phase precipitates during hot rolling of duplex stainless steel, a chromium-deficient layer is formed around it and the corrosion resistance of the steel is lowered. Similarly, even if chromium nitride precipitates, a chromium-deficient layer is formed around it, which reduces the corrosion resistance of steel. The prior art (Patent Documents 4 and 5) does not mention the relationship between the morphology of precipitates, the amount of precipitates, and other states of the metallic structure of steel and the corrosion resistance of steel.

Further, in the production of conventional duplex stainless steel hot-rolled steel sheets and clad steel sheets, solution heat treatment is indispensable. This is to eliminate intermetallic compounds and nitrides that reduce the corrosion resistance of the duplex stainless steel, as described above. In particular, duplex stainless steel, which is used as a cladding material for clad steel sheets, has the property that intermetallic compounds and nitrides tend to precipitate in the hot working temperature range. As a result, these precipitates are dispersed in the steel after hot rolling, resulting in deterioration of corrosion resistance. It is also possible to eliminate precipitates in the laminate by solution heat treatment. However, since solution heat treatment at 1000° C. or higher lowers the toughness of the base material, it is not preferable from the standpoint of the use of the clad steel sheet.
Furthermore, due to the demand for cost reduction and the demand for energy consumption reduction in recent years, it is desired to omit the solution treatment to reduce the clad steel plate manufacturing cost and the energy required for manufacturing.

The present invention uses a duplex stainless steel having a chemical composition with a Mo content of 0.5 to 2.5%, omits the solution heat treatment, and takes advantage of the properties (toughness and strength) of the steel plate that serves as the base material. An object of the present invention is to provide a clad steel sheet having good corrosion resistance and a method for manufacturing the same.

In order to solve the above-mentioned problems, the present inventors found that in the process of hot rolling the base material and the cladding material in the clad steel plate manufacturing process, an intermetallic compound and a nitriding compound are added to the duplex stainless steel as the cladding material. We thought that if no substances precipitated, the corrosion resistance would not be impaired even if the subsequent solution heat treatment was omitted. Therefore, we thought of finding a solution by using duplex stainless steel, which can maintain high corrosion resistance even when the hot rolling temperature is lowered, as the cladding material of the clad steel plate. In order to obtain such a duplex stainless steel, the chemical composition of the hot-rolled steel material without solution heat treatment, the hot working conditions and the amount of precipitates such as sigma phase and chromium nitride, and the recovery and regeneration of ferrite and austenite phases are required. We have obtained knowledge about the state of the metallographic structure including crystals, as well as the relationship between impact properties and corrosion resistance of steel materials. In particular, since the sigma phase tends to precipitate in the ferrite phase, the above problem is solved by controlling the hot rolling temperature so that the micro strain _εα in the ferrite phase satisfies a specific relationship with Tσ. I found a solution.
Based on these findings, the inventors have completed the present invention regarding a clad steel sheet that can omit the solution heat treatment even when stainless steel having a chemical composition with a Mo content of 0.5 to 2.5% is used as a cladding material. .

That is, the gist of the present invention is as follows.
[1]
A clad steel plate in which one or both sides of a steel plate serving as a base material are clad with a cladding material,
The chemical composition of the laminated material is % by mass,
C: 0.030% or less,
Si: 0.05 to 1.00%,
Mn: 0.10-3.00%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 22.0 to 27.0%,
Ni: 4.00 to 7.00%,
Mo: 0.50-2.50%,
W: 0 to 1.50%,
N: 0.100 to 0.250%,
Oxygen: 0.001-0.006%,
Co: 0 to 1.00%,
Cu: 0 to 3.00%,
V: 0 to 1.00%,
Nb: 0 to 0.200%,
Ta: 0 to 0.200%,
Ti: 0 to 0.030%,
Zr: 0 to 0.050%,
Hf: 0 to 0.100%,
B: 0 to 0.0050%,
Al: 0 to 0.050%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0-0.100%,
Sn: contains 0 to 0.100%,
the balance being Fe and impurities,
PREW obtained by Equation 5 is 24 or more and 35 or less,
The σ phase precipitation temperature Tσ (° C.) obtained by Equation 2 is 800° C. or higher and 950° C. or lower,
The surface hardness of the laminated material is 1.3 times or less that of the solution heat treated state,
A clad steel sheet, wherein the micro strain ε _α of the ferrite phase is equal to or less than ε _max obtained by Equation (1).
ε _max =0.0035−Tσ × 2.63 × 10 ⁻⁶ (Formula 1)
Tσ = 4Cr + 25Ni + 71 (Mo + W) - 11.4 (Mo - 1.3) × (Mo - 1.3) + 5Si - 6Mn - 30N + 569 (°C) (Formula 2)
PREW=Cr+3.3(Mo+0.5W)+16N (Formula 5)
However, each element symbol in Formulas 1, 2, and 5 indicates the content (% by mass) of the element in the cladding material, and 0 is substituted when the element is not contained.
[2]
In the chemical composition of the laminated material, in mass%,
Co: 0.03 to 1.00%,
Cu: 0.30 to 3.00%,
V: 0.03 to 1.00%,
Nb: 0.010 to 0.200%,
Ta: 0.010 to 0.200%,
Ti: 0.003 to 0.030%,
Zr: 0.005 to 0.050%,
Hf: 0.010 to 0.100%,
B: 0.0005 to 0.0050%,
Al: 0.003 to 0.050%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0050%,
REM: 0.010 to 0.100%,
Sn: The clad steel sheet according to [1] containing one or more of 0.010 to 0.100%.
[3]
Two slabs in which the laminated material described in [1] or [2] is laminated on one side of a steel plate as a base material are superimposed so that the laminated material is arranged on the inside to form a single slab and hot rolled. A method for manufacturing a clad steel plate by sandwich assembly,
[1] or [2] characterized in that hot rolling is performed to a finishing temperature TF that satisfies Equation 3, and then cooling from TF to 600 ° C. at an average cooling rate of 0.6 ° C./s or more The method for producing the clad steel plate according to 1 .
TF≧Tσ−50 (° C.) (Formula 3)
Tσ = 4Cr + 25Ni + 71 (Mo + W) - 11.4 (Mo - 1.3) × (Mo - 1.3) + 5Si - 6Mn - 30N + 569 (°C) (Formula 2)
However, each element symbol in Formula 2 represents the content (% by mass) of the element in the cladding material, and 0 is substituted when the element is not contained.
[4]
A method for manufacturing a clad steel plate by open-sand assembly in which a slab obtained by laminating the laminated material according to [1] or [2] to one or both sides of a steel plate as a base material is hot-rolled, wherein the clad steel plate satisfies formula 4. The clad according to [1] or [2], wherein hot rolling is performed so that the finishing temperature TF is reached, and then the average cooling rate from TF to 600 ° C is cooled at 0.6 ° C / s or more. A method of manufacturing a steel plate.
TF≧Tσ+30 (°C) (Formula 4)
Tσ = 4Cr + 25Ni + 71 (Mo + W) - 11.4 (Mo - 1.3) × (Mo - 1.3) + 5Si - 6Mn - 30N + 569 (°C) (Formula 2)
However, each element symbol in Formula 2 represents the content (% by mass) of the element in the cladding material, and 0 is substituted when the element is not contained.

According to the present invention, it is possible to obtain a clad steel sheet having good corrosion resistance while omitting the solution heat treatment and taking advantage of the properties (toughness and strength) of the steel sheet serving as the base material. As a result, the contribution to industry and the environment is extremely large.

FIG. 1 is a diagram for explaining the relationship between the micro strain of the ferrite phase in the surface layer of the cladding material and the σ phase precipitation temperature: Tσ (° C.).

An embodiment of the present invention will be described below. Unless otherwise specified, "%" regarding components indicates mass % in steel.

As described above, in order to solve the above-mentioned problems, the present inventors have found that in the process of hot rolling the base material and the cladding material in the process of manufacturing the clad steel plate, the duplex stainless steel as the cladding material is used. If intermetallic compounds and nitrides do not precipitate in the steel, the corrosion resistance will not be impaired even if the subsequent solution heat treatment is omitted. Therefore, we considered using duplex stainless steel, which can maintain high corrosion resistance even when the hot-rolling temperature is lowered, as the cladding material of the clad steel plate. In order to obtain such a duplex stainless steel, the chemical composition of the hot-rolled steel material without solution heat treatment, the hot working conditions and the amount of precipitates such as sigma phase and chromium nitride, and the recovery and regeneration of ferrite and austenite phases are required. The following experiments were carried out to investigate the state of the metallographic structure including crystals, etc., and the relationship between the corrosion resistance of steel materials.

A sigma phase precipitation temperature Tσ was introduced as an index for the precipitation of the sigma phase. A hot-rolled steel material with a plate thickness of 10 mm to 35 mm was obtained by setting the entry-side temperature TF of the finishing pass to 700 to 1000° C. and setting the accelerated cooling start temperature TC after the completion of hot rolling to 950° C. or less. The strength and impact properties of the obtained hot-rolled steel and the steel subjected to solution heat treatment were evaluated, and the metallographic structure and corrosion resistance of the surface layer and central portion of the plate thickness were evaluated.

Next, based on the findings of the duplex stainless steel obtained from the above experiments, an attempt was made to obtain steel compositions and manufacturing conditions that would allow high strength to be obtained even under rolling conditions in which the duplex stainless steel has good corrosion resistance. In other words, the following experiment was carried out considering that it is necessary to increase the recrystallization temperature by adjusting the chemical composition of ordinary steel and to control the cooling below the σ phase precipitation temperature range. A slab is created by attaching duplex stainless steel as a cladding material to the surface of steel containing various components. Assembled using This clad material was hot-rolled to obtain a clad steel plate with a laminate thickness of 3 mm and a total thickness of 20 mm to 50 mm, and the strength, impact properties, metallographic structure, and corrosion resistance were evaluated.
Through the above experiments, the inventors have completed the present invention regarding a clad steel sheet that uses a duplex stainless steel as a cladding material and can omit the solution heat treatment.

First, the chemical composition of the cladding material will be described.

The content of C is limited to 0.030% or less in order to ensure the corrosion resistance of stainless steel. If the Cr content exceeds 0.030%, Cr carbide is formed during hot rolling, degrading corrosion resistance and toughness.

Si is contained by 0.05% or more for deoxidation. Preferably, it should be contained in an amount of 0.20% or more. However, if the content exceeds 1.00%, the toughness deteriorates. Therefore, the upper limit is limited to 1.00%. Preferably, it should be contained in an amount of 0.70% or less.

Mn has the effect of increasing the austenite phase and improving the toughness, ensuring the toughness of the base metal and the weld zone, and also having the effect of lowering the nitride precipitation temperature TN. Preferably, it should be contained in an amount of 0.20% or more. However, if the content exceeds 3.00%, the corrosion resistance deteriorates. Therefore, the upper limit is limited to 3.00%. The content is preferably 2.50% or less, more preferably 2.00% or less.

P is an element that is unavoidably mixed from the raw material, and deteriorates the hot workability and toughness. Preferably, it should be contained in an amount of 0.030% or less.

S is also an element that is unavoidably mixed from raw materials, and deteriorates hot workability, toughness and corrosion resistance. Preferably, it is 0.0030% or less.

Cr is contained in an amount of 22.0% or more to ensure basic corrosion resistance. Preferably, it should be contained in an amount of 23.0% or more. On the other hand, if the content exceeds 27.0%, the ferrite phase fraction increases, impairing the toughness and corrosion resistance of the weld zone. Therefore, the Cr content is set to 27.0% or less. Preferably, it should be contained in an amount of 26.0% or less.

4.00% or more of Ni is contained in order to stabilize the austenite structure and improve corrosion resistance to various acids and toughness. By increasing the Ni content, it becomes possible to lower the nitride precipitation temperature. Preferably, it should be contained in an amount of 4.50% or more. On the other hand, Ni is an expensive alloy, and its content is set to 7.00% or less from the viewpoint of cost. Preferably, it should be contained in an amount of 6.50% or less.

Mo is a very effective element that enhances the corrosion resistance of stainless steel, and is contained in an amount of 0.50% or more. Preferably, it should be contained in an amount of 1.00% or more. In order to improve corrosion resistance, it is preferable to contain a large amount of N, but since it is an element that promotes the precipitation of the sigma phase, it is preferable to contain 2.50% or less. The content is preferably 2.30% or less, more preferably 2.00% or less.

N (nitrogen) is an effective element that forms a solid solution in the austenite phase and increases strength and corrosion resistance. For this reason, the content is 0.100% or more. Preferably, it should be contained in an amount of 0.150% or more. The solid solubility limit increases according to the Cr content, but if the Cr content exceeds 0.250%, the nitride precipitation temperature TN rises and Cr nitride precipitates during hot rolling, impairing toughness and corrosion resistance. The upper limit of the content is set to 0.250%. Preferably, it should be contained in an amount of 0.220% or less.

O (oxygen) is an unavoidable impurity and an important element that constitutes oxides, which are typical non-metallic inclusions, and an excessive content impairs toughness. Also, the formation of coarse cluster-like oxides causes surface flaws. Therefore, the upper limit is set to 0.006%. On the other hand, excessive deoxidation increases the cost, so the lower limit was made 0.001%.

The balance is Fe and impurities. Impurities are components that are mixed in during the manufacturing process of steel and remain unremoved.
Furthermore, instead of Fe, one or more of the following elements (W, Co, Cu, V, Nb, Ta, Ti, Zr, Hf, B, Al, Ca, Mg, REM, Sn) You may Since these elements do not have to be contained, the content range includes 0%.

W, like Mo, is an element that improves the corrosion resistance of stainless steel and may be contained. On the other hand, since it is an expensive element, it should be contained in an amount of 1.50% or less. Preferably, it should be 1.00% or less. When it is contained, the preferred content is 0.05% or more.

Co is an element effective for increasing the toughness and corrosion resistance of steel and is selectively contained. If the content exceeds 1.00%, the element is an expensive element and the effect commensurate with the cost cannot be exhibited, so the upper limit was set to 1.00%. When it is contained, the preferable lower limit is 0.03% and the upper limit is 0.50%.

Cu is an element that additionally increases the acid corrosion resistance of stainless steel and has the effect of improving the toughness, so it can be contained. If the content exceeds 3.00%, εCu precipitates beyond the solid solubility during hot rolling and causes embrittlement, so the upper limit was made 3.00%. When Cu is contained, the preferred lower limit is 0.30% and the upper limit is 2.00%.

V, Nb, and Ta are elements that form carbides and nitrides in steel, and can be added in trace amounts to additionally improve corrosion resistance. On the other hand, if a steel containing N contains a large amount of V, Nb, and Ta, carbonitrides are formed and the toughness is impaired, so the upper limit is regulated. Considering the effect of V, Nb, and Ta, and the alloy cost, the upper limits of these elements are set to 1.00%, 0.200%, and 0.200%, respectively. Preferred ranges for inclusion include lower limits of 0.03%, 0.010% and 0.010% and upper limits of 0.20%, 0.100% and 0.100%, respectively.

Ti, Zr, and Hf are elements that form nitrides and carbides in steel, and can be contained in trace amounts for the purpose of refining the crystal structure. Since Ti, Zr, and Hf have very strong nitride-forming abilities, a large amount of Ti, Zr, and Hf in steel containing N produces coarse nitrides and impairs toughness. Regulated. Considering the effect of Ti, Zr, and Hf and the alloy cost, the upper limits of each were set to 0.030%, 0.050%, and 0.100%. Preferred ranges for inclusion include lower limits of 0.003%, 0.005% and 0.010% and upper limits of 0.020%, 0.030% and 0.050%.

B is an element that forms nitrides and carbides in steel, has a low solid solubility in steel, and is an element that tends to segregate at grain boundaries. As its action, it improves hot workability. On the other hand, an excessive content forms coarse nitrides, impairing the toughness of the steel. Therefore, the upper limit of the content is set at 0.0050%. When it is contained, the preferable lower limit is 0.0005% and the upper limit is 0.0035%.

Al can be contained for deoxidizing steel. In addition, it is an element that forms nitrides, and an excessive content forms coarse nitrides, impairing the toughness, so the upper limit was set to 0.050%. The preferable content when it is included, the lower limit is 0.003% and the upper limit is 0.030%.

Ca and Mg can be included to improve the hot workability of the steel. An excessive content adversely affects hot workability, so the upper limit was set at 0.0050%. When it is contained, the preferable content range is 0.0005% as the lower limit and 0.0035% as the upper limit.

REM can be included to improve the hot workability of the steel. An excessive content adversely affects hot workability, so the upper limit was set at 0.100%. A preferred content range has a lower limit of 0.010% and an upper limit of 0.080%. Here, REM is the total content of lanthanoid rare earth elements such as La and Ce.

Sn is an element that additionally increases the acid corrosion resistance of steel and can be included for this purpose. On the other hand, an excessive content impairs the hot workability of steel, so the upper limit was set at 0.100%. When it is contained, the preferable lower limit is 0.010% and the upper limit is 0.080%.

PREW is an index for the pitting corrosion resistance of stainless steel, and is calculated by Equation 5 using the contents (%) of alloying elements Cr, Mo, W, and N. If the duplex stainless steel has a PREW of less than 24, it cannot exhibit corrosion resistance in a brackish water environment. bottom.
PREW=Cr+3.3(Mo+0.5W)+16N (Formula 5)
However, each element symbol in Formula 5 indicates the content (% by mass) of the element, and 0 is substituted when the element is not contained.

σ phase precipitation temperature: Tσ (°C) is an index determined by the chemical composition of the cladding material, represents the temperature at which the sigma phase begins to precipitate in equilibrium, and is a value obtained by thermodynamic calculation for the equilibrium diagram of the metal material. be. Thermodynamic calculations can be performed using commercially available software called Thermocalc@ and a thermodynamic database (FE-DATA version 6, etc.). This calculation was performed for various duplex stainless steels. The sigma phase is an intermetallic compound containing Fe, Cr, Mo, and W as main elements, and Mo and W promote precipitation in a duplex stainless steel in which the Fe and Cr contents are within a certain numerical range. Cr is the main element that precipitates the sigma phase, and Tσ changes depending on the amount of Cr. For this reason, the inventors created an equation (Equation 2) for determining the value of Tσ that can be applied within the composition range of the invention steel. For the purpose of obtaining a desired clad steel sheet by controlling the precipitation of the sigma phase during hot rolling, the lower limit of Tσ (Equation 2) was set to 800°C and the upper limit to 950°C. When Tσ is less than 800°C, precipitation of the sigma phase is suppressed, but the steel grade contains less Mo and Cr, making it difficult to obtain the desired corrosion resistance. Since it is difficult to suppress the sigma phase precipitation of , the above numerical range is determined.
Tσ = 4Cr + 25Ni + 71 (Mo + W) - 11.4 (Mo - 1.3) × (Mo - 1.3) + 5Si - 6Mn - 30N + 569 (°C) (Formula 2)
However, each element symbol in Formula 2 indicates the content (% by mass) of the element, and 0 is substituted when the element is not contained.

The surface hardness of the cladding material is a property that affects the surface properties of the clad steel plate, and is preferably high. The steel sheet of the present invention is a product that is cooled after hot rolling and is applied without solution heat treatment, and is characterized by having a hardness higher than that of a solution heat treated material. For this reason, it is specified that the surface hardness of the cladding material should be 1.3 times or less that of the solution heat treated state. The higher the hardness, the better, but the large strain introduced during hot rolling and remaining promotes the precipitation of intermetallic compounds, impairing the corrosion resistance of the surface of the clad steel plate. I decided.
Here, the state of solution heat treatment means the surface hardness when the same laminated material is subjected to solution heat treatment. For the measurement of this hardness magnification, the position (Sample A1) of the target surface layer portion (region from the surface to the depth of 0.1 to 0.5 mm in the thickness direction) of the laminated material made of duplex stainless steel was measured. In addition to polishing and measuring the Vickers hardness, the material (sample B1) obtained by subjecting this sample to solution heat treatment at 1050 ° C. was similarly subjected to hardness measurement, and the surface hardness of the laminated material was compared to the solution heat treatment state. The value of the ratio (hardness of sample A1/hardness of sample B1) is quantified. Preferably, the surface hardness of the laminated material (Sample A1) is 1.05 to 1.30 times the surface hardness of the solution heat treated (Sample B1). Although the sample A1 and the sample B1 are described as being the same sample, they are not necessarily the same. good too.

Micro strain of ferrite phase: ε _α is an important characteristic value that defines the laminated steel sheet. Duplex stainless steel is composed of ferrite phase and austenite phase, but their microstructural change behavior during hot working is very different. The strain introduced by hot working becomes dislocations inside the material, and the dislocations are reduced through the processes of recovery and recrystallization. The rate of dislocation density reduction in the austenite phase is small. On the other hand, the rate of dislocation density reduction in the ferrite phase is high. Based on such findings, the present inventors prepared clad steel sheets of duplex stainless steel under various hot rolling conditions, and observed the metallographic structure of the surface layer of the cladding material. As a result, the present inventors have found that if the duplex stainless steel is not subjected to appropriate hot rolling, the decrease in dislocation density of the ferrite phase is suppressed, and strain remains in the clad steel plate cladding. They also found that intermetallic compounds, which are difficult to observe with an optical microscope, precipitate in the ferrite phase during the cooling process after hot rolling, and correspondingly, the corrosion resistance is lowered. Furthermore, the inventors have also found that the micro strain of the ferrite phase increases during the process of such precipitation. By organizing these findings, it is possible to obtain the desired characteristics by controlling the micro strain of the ferrite phase in the surface layer of the cladding material so that it is smaller than ε _max obtained by Equation 1 (Fig. 1). .
ε _max =0.0035−Tσ×2.63×10 ⁻⁶ (Formula 1)
However, each element symbol in Formula 1 represents the content (% by mass) of the element, and 0 is substituted when the element is not contained.

Here, the value of micro strain is a value that can be obtained by an X-ray diffraction method, and the unit is dimensionless. A specific measuring method will be described. The position (specimen A2) of the surface layer portion (the region from the surface to the depth of 0.1 mm or more and 0.5 mm or less in the thickness direction) of the laminated material made of duplex stainless steel was machined and electropolished to determine the strain during sample preparation. After finishing it to a size of about 3 mm thick x 20 mm wide x 20 mm long so that no remains, X-ray diffraction is performed using a radiation source such as CuKα rays, and the diffraction intensity profile A of each diffraction surface of the ferrite phase and austenite phase to measure. As a comparative material, the above laminated material was subjected to solution heat treatment at 1050 ° C. to remove the strain introduced by hot working (sample B2), and a similar X-ray diffraction sample was prepared and X-ray diffraction was performed. Then, a diffraction intensity profile B without distortion is measured. In a sample with a large residual strain, the diffraction intensity profile has a broadening (half-value width) with respect to the diffraction angle 2θ. This quantifies the microstrains of the ferrite and austenite phases. There is a certain relationship between the micro strain obtained in this way and the dislocation density inside the material. See Non-Patent Document 1 for the relationship between the two regarding the ferrite phase.

A manufacturing method will be described.

Sandwich assembly is a method of assembling two slabs, which are made by laminating a cladding material having the above-mentioned chemical composition on one side of a steel plate as a base material, and assembling them as a single slab by stacking them so that the cladding material is placed inside. is. The chemical composition of the steel plate that serves as the base material is not particularly limited. A slab consisting of a base material, a cladding material, a cladding material and a base material is heated according to a conventional method, extracted from a heating furnace, and then hot-rolled to produce a rolled clad steel sheet. The finishing temperature TF is defined as the steel material surface temperature at the entrance of the final pass of hot rolling. In the experiment described above, the relationship between the sigma phase precipitation temperature Tσ and TF of the two-phase stainless steel, which is the laminated material, is hot-rolled according to the relationship shown in Equation 3, and the TF of the clad steel plate after hot rolling from TF to 600 ° C. The average cooling rate of 0.6 ° C. / s (s indicates seconds) or more.
TF≧Tσ−50 (° C.) (Formula 3)
Tσ is the σ phase precipitation temperature obtained by Equation 2 above.
In the case of sandwich assembly, since the laminated material is arranged inside and the base material is arranged outside, the temperature of the laminated material is higher than the surface temperature of the steel plate.

The reason why the average cooling rate from TF to 600 ° C. is specified is that in the metal structure after hot rolling, the temperature range where intermetallic compounds such as sigma phase precipitate is about 900 (preferably Tσ) to 700 ° C. This is because it is necessary to increase the cooling rate in this temperature section. Preferably, the cooling start temperature is Tσ° C. or higher.
In order to make the cooling rate of 0.6° C./s or higher for a clad steel sheet hot-rolled by the sandwich assembly method having a large thickness, it is preferable to apply water cooling after completion of hot rolling. If the plate thickness is small, air cooling or forced air cooling may be used.
A higher cooling rate is better, preferably 1.0° C./s or more, more preferably 5.0° C./s or more.

Open sand assembly is a method of assembling a slab by laminating a steel plate as a base material and a duplex stainless steel laminate. The slab composed of the base material and the clad material is heated according to a conventional method, extracted from the heating furnace, and then hot-rolled to produce a rolled clad steel sheet. The finishing temperature TF is defined as the steel material surface temperature at the entrance of the final pass of hot rolling. In the experiment described above, the relationship between the sigma phase precipitation temperature Tσ and TF of the duplex stainless steel, which is the cladding material, is hot-rolled according to the relationship shown in Equation 4, and the TF of the clad steel plate after hot rolling from TF to 600 ° C. It is preferable to manufacture so that the average cooling rate of is 0.6° C./s or more.
TF≧Tσ+30 (°C) (Formula 4)
Tσ is the σ phase precipitation temperature obtained by Equation 2 above.
In the case of open sandwich assembly, since the laminated material is arranged on the outside and the base material is arranged on the inside, it is considered that the temperature of the laminated material becomes the surface temperature of the steel plate.

The reason why the average cooling rate from TF to 600 ° C. is specified here is that, as in the case of sandwich assembly, in the metal structure after hot rolling, the temperature range where intermetallic compounds such as sigma phase precipitate is 900 (preferably Tσ) to about 700° C., it is necessary to increase the cooling rate in this temperature interval. Preferably, the cooling start temperature should be Tσ or higher.
A higher cooling rate is better, preferably 1.0° C./s or more. If the clad steel plate hot-rolled by the open-sand assembly method is cooled at a high cooling rate, such as water cooling, the clad steel plate will warp due to the difference in thermal expansion between the cladding material and the base steel, making it difficult to thread. It can be difficult. Therefore, it is preferable to adopt a cooling method such as air cooling or forced air cooling. Preferably, it should be limited to 10.0° C./s or less.

Appropriate hot rolling is performed according to the sigma phase precipitation temperature of the duplex stainless steel used as the cladding material to suppress structural changes due to sigma phase precipitation on the surface of the cladding material, producing clad steel sheets with excellent corrosion resistance. can. When the duplex stainless steel is hot rolled at a temperature supercooled below the sigma phase precipitation temperature, the structural change of the duplex stainless steel progresses according to the degree of supercooling, and the corrosion resistance decreases. Here, in the clad steel plate of the sandwich assembly method, since the clad material is positioned inside the steel material, hot rolling is performed in a state where the temperature of the clad material is higher than the surface temperature of the steel material. Therefore, in the sandwich assembly method, the temperature of the laminated material is higher than the surface temperature of the steel material during hot rolling. Thus, the temperature at which the cladding material is substantially rolled differs depending on the assembling method. That is, it has become clear that the rolling temperature regulation of Equation 3 or Equation 4 is necessary for each assembly rolling method.

The clad steel sheet is obtained by setting the hot rolling temperature to a specific temperature or higher according to the sigma phase precipitation temperature of the duplex stainless steel that is the cladding material, and by setting the cooling rate to 0.6° C./s or higher. Therefore, the base material of the clad steel plate can be selected from one or more types selected from the group consisting of ordinary steel (carbon steel) and alloy steel (excluding stainless steel), and is not particularly limited. . It can be appropriately selected and used according to the intended use. Alloy steels include, but are not limited to, low alloy steels, nickel steels, manganese steels, chromium molybdenum steels, high speed steels, etc. Steels containing one or more elements in ordinary steels. can be

Examples are described below. Table 1 shows the chemical composition of the cladding material. Components other than those listed in Table 1 are Fe and unavoidable impurity elements. In addition, for the components shown in Table 1, the portion where the content is not described indicates the impurity level, REM means the lanthanoid rare earth element, and the content indicates the total of those elements. Also, the sigma phase precipitation temperature in the table was determined by Equation (2).

The clad steel plate is made of a duplex stainless steel having the chemical composition shown in Table 1, and the base material is C: 0.16%, Si: 0.21%, Mn: 0.63%, P: 0.018%. , S: 0.006%, Ni: 0.01%, Cr: 0.04%, Cu: 0.02%, and the balance Fe and impurities. A slab having a thickness of 130 mm was obtained by bonding a cladding material to one side of the steel as the base material. In the open sand method, this slab was used as the material for hot rolling. In the sandwich method, two slabs with a thickness of 130 mm were assembled by welding (sandwich assembly) with the cladding material on the inside to form a slab with a thickness of 260 mm.

In the hot rolling of the sandwich assembly method, a slab with a thickness of 260 mm was heated to a predetermined temperature of 1150 to 1220° C., and then a clad steel plate was produced by a two-stage rolling mill. As the hot rolling conditions, rolling was repeated 8 to 12 times, and finish rolling was performed at a TF of 830 to 1030° C. so that the final plate thickness was 20 to 50 mm. Some of the clad steel plates were accelerated cooled with a spray cooling device and then transferred to a cooling bed for cooling. After cooling, the plate was separated at the central portion of the plate thickness and separated into two clad steel plates. In this way, a clad steel plate having a cladding thickness of 3 mm and a total thickness of 10 to 25 mm was obtained. A part of this steel plate was subjected to solution heat treatment at 1050° C., and a sample for X-ray diffraction and a sample for pitting potential measurement were taken to evaluate the metal structure of the surface layer of the cladding material before and after the heat treatment.

In the hot rolling of the open sand assembling method, a clad steel plate was produced by a two-high rolling mill after heating a slab with the cladding side facing down to a predetermined temperature of 1150 to 1220°C. As for the hot rolling conditions, rolling was repeated 8 to 15 times, and finish rolling was performed at a TF of 780 to 1030° C. so that the final plate thickness was 10 to 25 mm. It was then transferred to a cooling bed and cooled. In this way, a clad steel plate having a cladding thickness of 3 mm and a total thickness of 10 to 25 mm was obtained.
A part of this steel plate was subjected to solution heat treatment at 1050° C., and a sample for X-ray diffraction and a sample for pitting potential measurement were taken to evaluate the metal structure of the surface layer of the cladding material before and after the heat treatment.

The micro strain on the surface of the laminated material was measured using a 2.5 mm x 20 w x 25 L X-ray diffraction sample that was finished by wet polishing and electrolytic polishing with emery paper so that the surface 0.3 mm below the surface of the steel material would not remain strained during processing of the test piece. was collected, and the diffraction profiles of the ferrite phase and the austenite phase were measured by X-ray diffraction measurement using a CuKα radiation source. The micro strains of both phases of sample A3 before solution heat treatment were obtained from the half-value width data of the steel materials before and after solution heat treatment (sample A and sample B). Among them, the values of the micro strain of the ferrite phase are shown in Table 2 (sandwich assembly method) and Table 3 (open sand assembly method).

The surface hardness of the laminated material was measured under the condition of a Vickers load of 5 kgf on the surface of the steel material 0.3 mm below the surface. Steel materials before and after solution heat treatment (i.e., sample A without solution heat treatment and sample B with solution heat treatment) are measured at n = 3, the average value is obtained, and the ratio of the average values (= sample Tables 2 and 3 show the values of (hardness of A/hardness of sample B).

The pitting potential measurement of the laminated material is performed by polarizing the surface of the steel material 0.3 mm below the surface in accordance with the method specified in JIS G0577 in a 50 ° C.-1 mol NaCl solution, and the current density is 100 μA / cm. ² was measured (V _C ′ ₁₀₀ ). The steel materials before and after solution heat treatment (i.e., sample A3 without solution heat treatment and sample B3 with solution heat treatment) were measured at n = 3, and the average value was determined to be the pitting potential of the sample. Tables 2 and 3 show the difference in the average values. Tables 2 and 3 also show the pitting potentials of Sample A and Sample B and the difference between them. If the pitting potential of sample A is 0.3 V or more, and the difference in the potential of the clad steel plate with respect to the potential of the solution heat-treated material (difference in pitting potential between sample A and sample B) is 0.1 V or less, it passes. and

Table 2 summarizes the examples of the clad steel plates assembled and hot-rolled by the sandwich method using the steels shown in Table 1 as the cladding members.
As described above, it was confirmed that the clad steel sheets according to the examples of the present invention have a large surface hardness of the laminated material, and the difference in corrosion resistance is 0.1 V or less compared to the solution heat treated material, which is comparable.

Table 3 summarizes the examples of the clad steel plate cladding, which was assembled by the open sand method and hot-rolled using the steel shown in Table 1 as the cladding.
As described above, it was confirmed that the clad steel sheets of the examples had a large surface hardness of the laminated material, and the difference in corrosion resistance was 0.1 V or less compared to the solution heat-treated material, which was comparable.

According to the present invention, it is possible to provide an economical clad steel sheet with reduced alloying elements and good corrosion resistance. can be used for

Claims (4)

A clad steel plate in which one or both sides of a steel plate serving as a base material are clad with a cladding material,
The chemical composition of the laminated material is % by mass,
C: 0.030% or less,
Si: 0.05 to 1.00%,
Mn: 0.10-3.00%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 22.0 to 27.0%,
Ni: 4.00 to 7.00%,
Mo: 0.50-2.50%,
W: 0 to 1.50%,
N: 0.100 to 0.250%,
Oxygen: 0.001-0.006%,
Co: 0 to 1.00%,
Cu: 0 to 3.00%,
V: 0 to 1.00%,
Nb: 0 to 0.200%,
Ta: 0 to 0.200%,
Ti: 0 to 0.030%,
Zr: 0 to 0.050%,
Hf: 0 to 0.100%,
B: 0 to 0.0050%,
Al: 0 to 0.050%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
REM: 0-0.100%,
Sn: contains 0 to 0.100%,
the balance being Fe and impurities,
PREW obtained by Equation 5 is 24 or more and 35 or less,
The σ phase precipitation temperature Tσ (° C.) obtained by Equation 2 is 800° C. or higher and 950° C. or lower,
The surface hardness of the laminated material is 1.3 times or less that of the solution heat treated state,
A clad steel sheet, wherein the micro strain ε _α of the ferrite phase is equal to or less than ε _max obtained by Equation (1).
ε _max =0.0035−Tσ × 2.63 × 10 ⁻⁶ (Formula 1)
Tσ = 4Cr + 25Ni + 71 (Mo + W) - 11.4 (Mo - 1.3) × (Mo - 1.3) + 5Si - 6Mn - 30N + 569 (°C) (Formula 2)
PREW=Cr+3.3(Mo+0.5W)+16N (Formula 5)
However, each element symbol in Formulas 1, 2, and 5 indicates the content (% by mass) of the element in the cladding material, and 0 is substituted when the element is not contained. In the chemical composition of the laminated material, in mass%,
Co: 0.03 to 1.00%,
Cu: 0.30 to 3.00%,
V: 0.03 to 1.00%,
Nb: 0.010 to 0.200%,
Ta: 0.010 to 0.200%,
Ti: 0.003 to 0.030%,
Zr: 0.005 to 0.050%,
Hf: 0.010 to 0.100%,
B: 0.0005 to 0.0050%,
Al: 0.003 to 0.050%,
Ca: 0.0005 to 0.0050%,
Mg: 0.0005-0.0050%,
REM: 0.010 to 0.100%,
Sn: The clad steel sheet according to claim 1, containing one or more of 0.010 to 0.100%. Two slabs obtained by laminating the laminated material according to claim 1 or 2 on one side of a steel plate as a base material are superimposed so that the laminated material is arranged on the inside to form a single slab, and hot rolling is performed. A method of manufacturing a clad steel plate by sandwich assembly, comprising:
3. The steel sheet according to claim 1 or 2, wherein hot rolling is performed so that the finishing temperature TF satisfies Equation 3, and then cooling is performed from TF to 600°C at an average cooling rate of 0.6°C/ s or more. clad steel plate manufacturing method.
TF≧Tσ−50 (° C.) (Formula 3)
Tσ = 4Cr + 25Ni + 71 (Mo + W) - 11.4 (Mo - 1.3) × (Mo - 1.3) + 5Si - 6Mn - 30N + 569 (°C) (Formula 2)
However, each element symbol in Formula 2 represents the content (% by mass) of the element in the cladding material, and 0 is substituted when the element is not contained. A method for manufacturing a clad steel plate by open sand assembly in which hot rolling is performed on a slab in which the laminated material according to claim 1 or 2 is bonded to one or both sides of a steel plate as a base material, the finish satisfying formula 4 The method for producing a clad steel sheet according to claim 1 or 2, wherein hot rolling is performed to a temperature TF, and then cooling is performed from TF to 600 ° C. at an average cooling rate of 0.6 ° C./s or more. .
TF≧Tσ+30 (°C) (Formula 4)
Tσ = 4Cr + 25Ni + 71 (Mo + W) - 11.4 (Mo - 1.3) × (Mo - 1.3) + 5Si - 6Mn - 30N + 569 (°C) (Formula 2)
However, each element symbol in Formula 2 represents the content (% by mass) of the element in the cladding material, and 0 is substituted when the element is not contained.

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